Color navigation system

ABSTRACT

A system and method for color navigation through a CIELAB color space with coordinates hue (H), lightness (L), chroma (C), with intervals ΔH, ΔL, ΔC such that the perceptual color difference AE1Nv, defined according to the formula: AZW =^{AL)2+{fc(H)−ACf+[AH)2, is constant for steps in direction C and L and for steps in direction H along boundary of color space. fc(H) is a proportionality factor between 0 and 1 depending on hue. For hue values between red (H 1 ) and green (H 2 ), fc(H) =1. For hue values between green (H 2 ) and blue (H 3 ), the proportionality factor fc(H) is set to a constant value foB such that Nκ,GB(fGB) differs less than 50% from NH,RG—For hue values between blue (H 3 ) and red (H 1 ), the proportionality factor fc(H) is set to a constant value fβR such that N^BR^BR) differs less than 50% from NH,RG—.

This application is a national stage application under 35 U.S.C. §371 ofInternational Application No. PCT/IB2007/052430 filed on Jun. 22, 2007,and published in the English language on Jan. 3, 2008, as InternationalPublication No. WO/2008/001290, which claims priority to EuropeanApplication No. 06116110.5, filed on Jun. 27, 2006, incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention relates in general to the field of lighting. Moreparticularly, the present invention relates to a color navigation systemfor generating light with a variable color.

BACKGROUND OF THE INVENTION

Illumination systems for illuminating a space with a variable color aregenerally known. Generally, such systems comprise a plurality of lightsources, each light source emitting light with a specific color, therespective colors of the different light sources being mutuallydifferent. The overall light generated by the system as a whole is thena mixture of the light emitted by the several light sources. By changingthe relative intensities of the different light sources, the color ofthe overall light mixture can be changed.

It is noted that the light sources can be of different type, such as forinstance TL lamp, halogen lamp, LED, etc. In the following, simply theword “lamp” will be used, but this is not intended to exclude LEDs.

By way of an example of a variable color illumination system, anillumination system in a theatre is mentioned. During a show, it may bedesirable to change the color of the lighting. However, also in the caseof homes, shops, restaurants, hotels, schools, hospitals, etc., it maybe desirable to be able to change the color of the lighting. In the caseof a theatre or the like, the colors are typically changed with a viewto enhance dramatic effects, but in other situations it may be moredesirable to have smooth and slow transitions.

As should be clear to a person skilled in the art, the color of lightcan be represented by coordinates of a color point in a color space. Insuch representation, changing a color corresponds to a displacement fromone color point to another color point in the color space, or adisplacement of the setting of the color point of the system. Further, asequence of colors corresponds to a collection of color points in thecolor space, which collection will be indicated as a path. Dynamicallychanging the colors can then be indicated as “traveling” such path. Morein general, dynamically changing the colors of lighting will beindicated as “navigating” through the color space.

Typically, an illumination system comprises three lamps. Usually, theselamps are close-to-red (R), close-to-green (G), close-to-blue (B), andthe system is indicated as an RGB system. For each lamp, the lightintensity can be represented as a number from 0 (no light) to 1 (maximumintensity). A color point can be represented by three-dimensionalcoordinates (ξ1, ξ2, ξ3), each coordinate in a range from 0 to 1corresponding in a linear manner to the relative intensity of one of thelamps. The color points of the individual lamps can be represented as(1,0,0), (0,1,0), (0,0,1), respectively. These points describe atriangle in the color space. All colors within this triangle can begenerated by the system.

It is desirable to have a color navigation system, allowing a user tonavigate through the color space in a comfortable and intuitive way.Specifically, it is desirable that a color navigation system allows theuser to take such steps in the color space such that the perceived colorchange is constant.

In theory, the color space can be considered as being a continuum. Thiswill allow a user to select every possible color within theabove-mentioned triangle. However, calculating the stepsize in a certainstep direction such that the perceived color change has a certain valuerequires the use of rather powerful and costly microprocessors.Therefore, it is more advantageous for a color navigation system to havea color table, comprising predefined color points. Navigating throughcolor space then translates to making steps from one color point in thetable to the next color point in the table. Navigation can then beperformed easily under the control of a simple user interface. Anexample of such simple user interface comprises six buttons, two buttons(step-up and step-down, respectively) for each color coordinate.Actuating one of these buttons will result in a step along thecorresponding color coordinate axis, the step resulting in a predefinedcolor perception difference.

A problem in this respect is that the RGB color space is not a linearspace, so that, when taking a discrete step of a certain size along oneof the color intensity coordinate axes, the amount of color changeperceived by the user is not constant but depends on the actual positionwithin the color space.

In order to solve this problem, different representations of the colorspace have been proposed, such as the CIELAB color space, where theindependent variables are hue (H), saturation (S; in CIELAB calculatedwith S=Chroma/Lightness), brightness (B; in CIELAB calculated fromLightness). Because of the perceptual uniformity of Lightness (i.e. alinear change of Lightness level is also perceived as a linear change oflight intensity level by the user), it is advantageous to use thisparameter instead of Brightness. However, to generalize the descriptionthe parameter “Brightness” will be used in the explanation next, whichvalues are also described with a perceptual uniform distribution (e.g.in u′V′Y space, with “Y” describing intensity, perceptual uniformBrightness distribution is logarithm(Y)). The CIELAB color space can beseen as a three-dimensional space of discrete points (3D grid). Eachpoint in this space can be represented by coordinates m, n, p, and ineach point the hue (H), saturation (S), Brightness (B) have specificvalues H(m,n,p), S(m,n,p), B(m,n,p), respectively. A user can take adiscrete step along any of the three coordinate axes, resulting inpredefined and constant changes in hue, saturation or Brightness,respectively, as long as the color is inside the outer boundary of thecolor space (color gamut). In principle, the variables hue, saturationand Brightness are independent from each other.

A problem is now to define a good navigation algorithm, definingnavigation steps such that the perceived color change is constant, inorder to build a suitable table or in order to step through an existingtable. According to the CIELAB theory, when a step is made from a firstcolor point with Chroma value C1 and hue angle h1 to a second colorpoint with Chroma value C2 and hue angle h2, the color difference ΔE isdefined asΔE=√{square root over (ΔL ² +ΔC ² +ΔH ²)}in which ΔH is the metric Hue difference, defined as ΔH= C·Δhwith C being the average of C1 and C2 and with Δh=h2−h1.

Although the above formula is precise according to theory, it appearsnot to be precise in people's actual perception. Thus, there is a needfor an improved color difference formula that is better adapted topersonal perception.

In the article “Comparative Analysis of the Quantization of Color Spaceson the Basis of the CIELAB Color-difference Formula” in ACM transactionson Graphics, Vol. 16, nr.2, April 1997, p. 109-154, B. Hill et aldescribe an improved color difference formula:

${\Delta\; E_{94}^{\star}} = \sqrt{\left( \frac{\Delta\; L^{\star}}{k_{L}S_{L}} \right)^{2} + \left( \frac{\Delta\; C_{ab}^{\star}}{k_{C}S_{C}} \right)^{2} + \left( \frac{\Delta\; H_{ab}^{\star}}{k_{H}S_{H}} \right)^{2}}$where ΔL*, ΔC*_(ab), and ΔH*_(ab) are the CIELAB 1976 color differencesof lightness, chroma, and hue, respectively;where k_(L), k_(C), and k_(H) are factors to match the perception ofbackground conditions;and where S_(L), S_(C), and S_(H) are linear functions of C*_(ab).

According to the article, standard values have been assumed as follows:k_(L)=k_(C)=k_(H)=1S_(L)=1S _(C)=1+0.045·C* _(ab)S _(H)=1+0.015·C* _(ab)

However, experiments of the present inventor have shown that the aboveformula does not give satisfying results. Thus, a first objective of thepresent invention is to provide a color difference formula giving moresatisfying results in practical experiments.

A further aspect of the problem relates to the fact that the boundary ofthe color space is usually not a circle but has the general shape of atriangle with curved sides. In the case of saturated LED primaries, theblue primary creates a long elongated corner in the triangle, and theRED-GREEN side is relatively short. When a user navigates along thisboundary in the hue direction, taking steps such that the perceivedcolor difference ΔE in accordance with the above-mentioned formula ismaintained constant, he will find that there are far more steps in thecyan-blue-magenta region than in the red-green region. This is not anattractive situation. This situation is caused by the fact that, whennavigating along the boundary of the color space, it is not possible tokeep both the lightness and the chroma constant, and differences inlightness and chroma are considered to contribute to perceived colordifferences. When navigating close to blue, differences in chroma arerelatively large so that, when the perceived color difference ismaintained constant, the steps in hue are relatively small.

Thus, it is a further objective of the present invention to provide asolution for this problem as well.

More particularly, the present invention aims to provide a method forcalculating color steps resulting in a good compromise between thedesire to have the steps being equidistant in perceived colordifferences on the one hand, and the desire to have substantiallycomparable numbers of color steps along the boundary of the color spaceon the other hand.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a color differenceformula is defined as follows:ΔE _(INV)=√{square root over ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square rootover ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square root over ((ΔL)²+(f_(C)(H)·ΔC)²+(ΔH)²)}wherein ΔE_(INV) indicates the color difference according to the presentinvention,and wherein f_(C)(H) is a factor between 0 and 1 which only depends onhue H. This factor is suitably chosen such as to mitigate the effect ofchroma on the perceived color difference.

According to a second aspect of the present invention, the color spaceis divided in three sectors RED-GREEN, GREEN-BLUE, and BLUE-RED, andvalues for f_(C)(H) are defined such that the numbers of color pointsalong the color space boundary in the hue direction for said threesectors are mutually equal, or at least do not differ more than 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the presentinvention will be further explained by the following description withreference to the drawings, in which same reference numerals indicatesame or similar parts, and in which:

FIG. 1 schematically shows a chromaticity diagram;

FIG. 2 schematically shows a CIELAB plot;

FIG. 3 is a block diagram schematically illustrating a color navigationsystem.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows a CIE(xy) chromaticity diagram. This diagramis well-known, therefore an explanation will be kept to a minimum.Points (1,0), (0,0), and (0,1) indicate ideal red, blue and green,respectively, which are virtual colors. The curved line 1 represents thepure spectral colors. Wavelengths are indicated in nanometers (nm). Adashed line 2 connects the ends of the curved line 1. The area 3enclosed by the curved line 1 and dashed line 2 contains all visiblecolors; in contrast to the pure spectral colors of the curved line 1,the colors of the area 3 are mixed colors, which can be obtained bymixing two or more pure spectral colors. Conversely, each visible colorcan be represented by coordinates in the chromaticity diagram; a pointin the chromaticity diagram will be indicated as a “color point”.

It is noted that a different graphical color representation, forinstance the RGB diagram, may also be used, as should be clear to aperson skilled in this art. However, the distribution of colors in theRGB space is completely device-dependent (e.g. a certain RGB value willin general give different perceived colors with different lamps thateach have different RGB primary colors).

Preferably, the colors are represented in a device-independent colorspace, like the CIELAB color space, also referred to as the L*a*b* colorspace. FIG. 2 is a diagram illustrating the CIELAB space. The CIELABspace is preferred due to its perceptual uniformity; however, asmentioned above, it can clearly be seen that the CIELAB curve isdistorted in comparison to the CIE(xy) diagram, with quite a large blue“tail”.

Since the color definitions associated with these color spaces are knownto persons skilled in this art, an extensive explanation will be omittedhere. It suffices to mention that these spaces have hue (this will beexplained hereinafter), saturation (this will be explained hereinafter),and brightness (a measure for the overall light intensity) asindependent variables, and that a color representation in RGB space canbe converted to a color representation in CIELAB color space, or viceversa, via one-to-one matrix transformations.

The basic concepts of Hue, Saturation and Brightness are most easilyexplained in the CIE 1931 (x,y) color space, although in other colorspace other definitions can be obtained. For simplicity, we use CIE 1931(x,y) color space next.

When two pure spectral colors are mixed, the color point of theresulting mixed color is located on a line connecting the color pointsof the two pure colors, the exact location of the resulting color pointdepending on the mixing ratio (intensity ratio). For instance, whenviolet and red are mixed, the color point of the resulting mixed colorpurple is located on the dashed line 2. Two colors are called“complementary colors” if they can mix to produce white light. Forinstance, FIG. 1 shows a line 4 connecting blue (480 nm) and yellow (580nm), which line crosses a white point, indicating that a correctintensity ratio of blue light and yellow light will be perceived aswhite light. The same would apply for any other set of complementarycolors: in the case of the corresponding correct intensity ratio, thelight mixture will be perceived as white light. It is noted that thelight mixture actually still contains two spectral contributions atdifferent wavelengths.

If the light intensity of two complementary colors (lamps) is indicatedas I1 and I2, respectively, the overall intensity Itot of the mixedlight will be defined by I1+I2, while the resulting color will bedefined by the ratio I1/I2. For instance, assume that the first color isblue at intensity I1 and the second color is yellow at intensity I2. IfI2=0, the resulting color is pure blue, and the resulting color point islocated on the curved line 1. If I2 is increased, the color pointtravels the line 4 towards the white point. As long as the color pointis located between pure blue and white, the corresponding color is stillperceived as blue-ish, but closer to the white point the resulting colorwould be paler.

In the following, the word “color” will be used for the actual color inthe area 3, in association with the phrase “color point”. The“impression” of a color will be indicated by the word “hue”; in theabove example, the hue would be blue. It is noted that the hue isassociated with the spectral colors of the curved line 1; for each colorpoint, the corresponding hue can be found by projecting this color pointonto the curved line 1 along a line crossing the white point.

Further, the fact whether a color is a more or less pale hue will beexpressed by the phrase “saturation”. If a color point is located on thecurve 1, the corresponding color is a pure spectral color, alsoindicated as a fully saturated hue (saturation=1). As the color pointtravels towards the white point, the saturation decreases (lesssaturated hue or paler hue); in the white point, the saturation is zero,per definition.

It is noted that many visible colors can be obtained by mixing twocolors, but this does not apply for all colors, as can easily be seenfrom FIG. 1. In order to be able to produce light having any desiredcolor, three lamps producing three different colors are needed. Morelamps may be used, but that is not necessary.

FIG. 3 schematically shows a block diagram of an illumination system 10,comprising a lamp assembly 14. The lamp assembly 14 comprises aplurality (here: three) of lamps 12A, 12B, 12C, each with an associatedlamp driver 13A, 13B, 13C, respectively, controlled by a commoncontroller 15. A user input device is indicated at 19. The three lamps12A, 12B, 12C generate light 16A, 16B, 16C, respectively, with mutuallydifferent light colors; typical colors used are red (R), green (G), blue(B). Instead of pure red, green and blue, the lamps will typically emitlight close-to-red, close-to-green and close-to-blue, as indicated bythree exemplary color points C1, C2, C3 in FIG. 1, respectively. Theoverall light emitted by the lamp assembly 14 is indicated at 17; thisoverall light 17, which is a mixture of individual lights 16A, 16B, 16C,has a color point within the triangle defined by corner points C1, C2,C3. With the system 10, it is possible to set the mixture color of theoutput light mixture 17 at any desired location within said triangle, ifit is possible to change the light intensities of the individual lamps12A, 12B, 12C continuously. Typically, however, the controller 15 is adigital controller, and the light intensities of the individual lamps12A, 12B, 12C can only be changed with discrete steps. In such case, theattainable color points are located along a grid in the color space. Asregards color representation, CIELAB color space is preferred, becausethe distance between two neighboring grid points corresponds tosubstantially equal differences in perceived color over the entireCIELAB color space.

In the CIELAB color space, the hue, saturation and brightness can bevaried independently from each other, as long as the color is inside thecolor space boundary. In the present invention, we use linear axes forHue, Saturation and Lightness; these linear axes span the color spacewith cylindrical coordinates. Further, each axis is discretized, i.e. itis only possible to take discrete steps along each axis, respectively.Those steps are chosen such that corresponding steps in perceived colorΔE_(INV) are uniform. Here, the perceived color step is definedaccording to the following formula:ΔE _(INV)=√{square root over ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square rootover ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square root over ((ΔL)²+(f_(C)(H)·ΔC)²+(ΔH)²)}  (1)wherein f_(C)(H) is a proportionality factor between 0 and 1 which onlydepends on hue H, as will be described later. Thus, the color space isfilled with a grid of color points, wherein the distance betweenneighboring color points in the L-direction is indicated as ΔL, thedistance between neighboring color points in the C-direction isindicated as ΔC, and the distance between neighboring color points inthe H-direction is indicated as ΔH. It is noted that C indicates chroma,defined as C=S·L, S indicating saturation and L indicating lightness.

According to the invention proposal, the distribution of lightnesslevels is equidistant. Particularly, the lightness L can be varied froma minimum value Lmin (usually taken larger than zero) to a maximum valueLmax in equidistant steps. The number of possible lightness levels willbe indicated by N_(L). Then, using a “lightness index” p, the N_(L)possible values of lightness L(p) can be expressed according to thefollowing formula:L(p)=Lmin+p*ΔL  (2)wherein index p is an integer from 0 to N_(L)−1.

It can easily be seen that ΔL=(Lmax−Lmin)/(N_(L)−1). When stepping inthe L-direction, ΔE_(INV)=ΔL applies. It is noted that a linear increaseof lightness is also perceived by human observers as a linear increaseof brightness.

Further according to the invention proposal, the distribution ofsaturation levels is equidistant. Particularly, the saturation S can bevaried from a minimum value Smin (usually equal to zero) to a maximumvalue Smax (usually equal to one) in equidistant steps. The number ofpossible saturation levels will be indicated by N_(S). Then, using a“saturation index” n, the N_(S) possible values of saturation S(n) canbe expressed according to the following formula:S(n)=Smin+n*ΔS  (3)wherein index n is an integer from 0 to N_(S)−1.

It can easily be seen that ΔS=(Smax−Smin)/(N_(S)−1).

In view of the fact that C=S·L, when stepping in the S-direction,ΔH=ΔL=0, therefore

ΔE_(INV)=f_(C)(H)·ΔC=f_(C)(H)·ΔS·L applies. Thus, the perceived colordifferences are constant when stepping in the S-direction, but the sizeof these differences does depend on H and L.

Regarding the hue difference between two color points having chromavalues C1 and C2, respectively, and having hue angles h1 and h2,respectively, it is noted that, in CIELAB, the metric Hue difference ΔHis used, defined along a hue circle around the color space boundarywith:ΔH= C*Δh*with C* being the arithmetic mean of C1 and C2 and with Δh=h1−h2.(H2−H1) is the metric length of the hue circle segment along the colorspace boundary (which is calculated as the sum of all ΔH differencesbetween sequential colors along the boundary).

In prior art, the hue H is discretized over the entire color gamut froma suitably chosen minimum value Hmin to a suitably chosen maximum valueHmax in equidistant steps. As explained earlier, when navigating alongthe border of the color space taking steps of size ΔE, this results in arelatively large number of steps in the BLUE range (i.e. the differencein H between subsequent steps is relatively small in the BLUE range) anda relatively low number of steps in the range RED-GREEN. This isrecognized by the present inventor as being caused by the fact that thechroma C is given the same weight over the entire color space. Accordingto the present invention, discretization is done in a different way suchas to reduce this problem. According to the present invention, the colorspace is divided in three sectors 44, 45, 46, as illustrated in FIG. 2,and the weight of the chroma C is set separately in these three sectors44, 45, 46. In FIG. 2, a white point is indicated W. A line 41 ofconstant red hue connects white point W with a red color point; thecorresponding hue is indicated as H1. A line 42 of constant green hueconnects white point W with a green color point; the corresponding hueis indicated as H2. A line 43 of constant blue hue connects white pointW with a blue color point; the corresponding hue is indicated as H3. Afirst color sector 44, extending from RED to GREEN and thereforeindicated as the RED/GREEN sector, is defined between lines 41 and 42. Asecond color sector 45, extending from GREEN to BLUE and thereforeindicated as the GREEN/BLUE sector, is defined between lines 42 and 43.A third color sector 46, extending from BLUE to RED and thereforeindicated as the BLUE/RED sector, is defined between lines 43 and 41.The borderline of the color space is indicated as 47.

To define the Hue distribution, the following method is used. Thelightness L is set at a certain level, preferably the lightness valuefor which Cmax is found, with Cmax being the maximum value of chroma C;this lightness value will be indicated as L(Cmax). While keepinglightness L constant, i.e. ΔL=0, the borderline 47 is navigated withsteps of size ΔE_(INV) being constant. It should be clear that, witheach step, chroma and hue change according to the following formuladerived from formula 1:ΔE _(INV)=√{square root over ((f _(C)(H)·ΔC)²+(ΔH)²)}{square root over((f _(C)(H)·ΔC)²+(ΔH)²)}  (4)

With each such step, a color point is reached having a certain Hue valueH(m), m being a hue index. These values H(m) will be used as thediscretized values of the hue axis, i.e. the hue coordinates of the gridof color points.

In the RED/GREEN sector 44, the proportionality factor f_(C)(H) is setto be equal to 1, and the resulting number of steps from H1 (red) to H2(green) will be indicated by N_(H,RG).

In the GREEN/BLUE sector 45, the proportionality factor f_(C)(H) is setto a constant value f_(GB), and the resulting number of steps from H2(green) to H3 (blue) will be indicated by N_(H,GB)(f_(GB)), indicatingthat this number of steps depends on f_(GB).

In the BLUE/RED sector 46, the proportionality factor f_(C)(H) is set toa constant value f_(BR), and the resulting number of steps from H3(blue) to H1 (red) will be indicated by N_(H,BR)(f_(BR)), indicatingthat this number of steps depends on f_(BR).

In the prior art, f_(GB) and f_(BR) are equal to 1, but this has theproblem that it would result in large differences between N_(H,RG),N_(H,GB) and N_(H,BR). According to the invention, this problem issolved by selecting f_(GB) and f_(BR) smaller than 1, ideally such thatN_(H,RG)=N_(H,GB)(f_(GB))=N_(H,BR)(f_(BR)), but in any case such thatN_(H,GB)(f_(GB)) and N_(H,BR)(f_(BR)) differ less than 50% fromN_(H,RG). In a practical experiment, values f_(GB)=f_(BR)=0.5 proved tobe satisfactory.

Thus, the complete trajectory of the borderline is divided intoN_(H,RG)+N_(H,GB)(f_(GB))+N_(H,BR)(f_(BR)) steps. In other words, thenumber of possible hue levels, indicated as N_(H), is equal toN_(H,RG)+N_(H,GB)(f_(GB))+N_(H,BR)(f_(BR)). Those levels, which can beindicated by a hue index m, are not equidistant, but have mutualdistances ΔH(H), i.e. the size of the steps ΔH depends on the actualvalue of the hue, in such a way that, when stepping from one hue levelto the next along the borderline 47, the above formula 4 is obeyed.

Thus, the invention describes a particularly advantageous way ofdiscretizing the color space. Navigation through the colors that aregenerated by this discretization results in approximately perceptualequidistant color steps ΔE_(INV) along the axes of Saturation andBrightness as long as the colors are inside the color space boundary,and in approximately perceptual equidistant color steps ΔE_(INV) whenstepping in the hue direction along the color space boundary. Thus, thedistribution is not equidistant for hue within the color space boundary,but results in a better distribution along the color space boundary. Afurther advantage is that the overall number of steps in the huedirection can be smaller, thus the required memory space for the colortable can be reduced.

It should be noted that, when using the CIELAB color space, a certainreference white point has to be chosen. The preferred choice, based onthe experience of the inventor, is to choose a white point on theblackbody line (described with (x_ref,y_ref) in CIE1931 space) with thesame correlated color temperature (CCT) as the RGB luminaire with RGB=[11 1], i.e. the luminaire at full light output. Alternatively, anarbitary white point with a CCT between 2500 [K] and 6500 [K] can beused.

From the above, it follows that points in the color space can be definedby indices m, n, p, and the color in those points can be considered asbeing a function of 3 independent parameters m, n, p. FIG. 3 illustratesthat the user input device 19 allows the user to independently selectvalues for m, n and p; particularly, the user is allowed to take stepsalong the axes of Lightness, Saturation and Hue. The user input device19 is shown as a combination of three independent input devices 19H, 195and 19B, independently providing input values m, n, p for the controller15. On the basis of those input values m, n, p, the controller generatesa set of control signals (ξ1, ξ2, ξ3) for the drivers 13A, 13B, 13C ofthe lamp assembly 14.

The controller 15 is provided with a memory 18 which contains tables forhue, saturation and brightness, respectively. Using the userinstructions received from user interface 19, the controller 15 takesvalues for hue, saturation and brightness from said memory 18 andgenerates its control signals (ξ1, ξ2, ξ3) on the basis of these values.

The present invention provides two possible implementations. In a firstimplementation, the tables in the memory 18 are filled using theabove-described formulas. In that case, navigating through the colorspace on the basis of the tables from memory 18 always results inperceptually uniform color steps. Such implementation does not requireany specific adaptation of a microprocessor: when taking a step from onecolor point to the next, the controller 15 simply takes the nextinformation from the table. Thus, the invention is embodied in thecontents of the table in the memory.

In a second implementation, the table in the memory 18 may be filled inan arbitrary way (but with Hue values increasing monotonically withincreasing index ‘m’, Saturation values increasing monotonically withincreasing index ‘n’, and Lightness values increasing monotonically withincreasing index ‘p’), and the invention is embodied in the controller15. The user is allowed to enter a navigation command defining a certaincolor step of size ΔE_(INV) in a certain direction. The controller 15,on receiving the navigation command from the user, calculates thecorresponding values of ΔL, ΔC, ΔH, and then looks in the table for thecolor point closest to color point defined by the calculated values ofΔL, ΔC, ΔH.

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that several variations and modifications are possible within theprotective scope of the invention as defined in the appending claims.

For instance, in the example the color table 18 is associated with thecontroller 15. It is, however, also possible that the color table 18 isassociated with the user interface 19.

In the above, the present invention has been explained with reference toblock diagrams, which illustrate functional blocks of the deviceaccording to the present invention. It is to be understood that one ormore of these functional blocks may be implemented in hardware, wherethe function of such functional block is performed by individualhardware components, but it is also possible that one or more of thesefunctional blocks are implemented in software, so that the function ofsuch functional block is performed by one or more program lines of acomputer program or a programmable device such as a microprocessor,microcontroller, digital signal processor, etc.

1. A memory comprising discrete color points and suitable for use with acolor navigation system generating light with a variable color, thecolor points defined according to three-dimensional CIELAB coordinateshue (H), lightness (L), chroma (C), with predefined intervals ΔH betweenneighboring color points in the hue direction, with predefined intervalsΔL between neighboring color points in the lightness direction, and withpredefined intervals ΔC between neighboring color points in the chromadirection, respectively; wherein ΔL is equidistant over the color space;and wherein ΔS is equidistant over the color space, with ΔC=ΔS·L, Sindicating saturation; such that the perceptual color differenceΔE_(INV), defined according to the formula:ΔE _(INV)=√{square root over ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square rootover ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square root over ((ΔL)²+(f_(C)(H)·ΔC)²+(ΔH)²)} is substantially constant over the color space forsteps in the L direction and in the S direction; wherein f_(C)(H) is aproportionality factor between 0 and 1 only depending on hue H; wherein,for hue values H between a first hue value (H1) corresponding to red anda second hue value (H2) corresponding to green, the proportionalityfactor f_(C)(H) is equal to 1 and the number of color points in the huedirection in this range is equal to N_(H,RG); wherein, for hue valuesbetween the second hue value (H2) corresponding to green and a third huevalue (H3) corresponding to blue, the proportionality factor f_(C)(H) isset to a constant value f_(GB) such that the number N_(H,GB(fGB)) ofcolor points in the hue direction in this range differs less than 50%from N_(H,RG), N_(H,GB(fGB) being equal to N_(H,RG); wherein, for huevalues between the third hue value (H3) and the first hue value (H1),the proportionality factor f_(C)(H) is set to a constant value f_(BR)such that the number N_(H,BR(fBR)) of color points in the hue directionin this range differs less than 50% from N_(H,RG); and wherein ΔH is afunction of H such that, for steps in the hue direction along the borderof the color space, said perceptual color difference ΔE_(INV) issubstantially constant.
 2. A user input device, comprising a memoryaccording to claim
 1. 3. A color navigation system for generating lightwith a variable color, comprising: a memory according to claim 1; a lampassembly capable of generating light with a variable color; a controllerfor controlling the lamp assembly; and a user input device coupled tothe controller; wherein the controller is configured, on the basis ofdata (m,n,p) received from the user input device and on the basis of theinformation in the memory to generate color control signals (ξ1, ξ2, ξ3)for the lamp assembly, such that the color point of the generated lightcorresponds to the user data.
 4. A color navigation system according toclaim 3, wherein the controller is configured to navigate through theCIELAB color space with steps ΔH in the hue direction, steps ΔL in thelightness direction, and steps ΔC in the chroma direction, or acombination of such steps; and the controller is configured, in responseto the user input data and taking into account a current color point, tocalculate a target color point based on said steps ΔH, ΔL, ΔC, to takefrom memory data relating to a color point closest to the target colorpoint, and to generate its color control signals (ξ1, ξ2, ξ3) for thelamp assembly on the basis of these data from memory.
 5. A colornavigation system according to the claim 3, wherein a white point (W) isan arbitrary white point with a correlated color temperature between2500 K and 6500 K.
 6. A color navigation system according to the claims3, wherein a white point (W) is selected on the blackbody line,described with (x_(REF),y_(REF)) in CIE1931 space, with the samecorrelated color temperature as the RGB luminaire with RGB=[1 1 1], i.e.the luminaire at full light output.
 7. A method for providing a memorydefining discrete color points in a color space, the method comprisingthe steps of: defining color points according to three-dimensionalCIELAB coordinates hue (H), lightness (L), chroma (C), with predefinedintervals ΔH between neighboring color points in the hue direction, withpredefined intervals ΔL between neighboring color points in thelightness direction, and with predefined intervals ΔC betweenneighboring color points in the chroma direction, respectively; whereinΔL is equidistant over the color space; wherein ΔS is equidistant overthe color space, with ΔC=ΔS·L, S indicating saturation, such that theperceptual color difference ΔE_(INV), defined according to the formula:ΔE _(INV)=√{square root over ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square rootover ((ΔL)²+(f _(C)(H)·ΔC)²+(ΔH)²)}{square root over ((ΔL)²+(f_(C)(H)·ΔC)²+(ΔH)²)}, is substantially constant over the color space forsteps in the L-direction and in the S-direction; wherein f_(C(H)) is aproportionality factor between 0 and 1 only depending on hue H; wherein,for hue values H between a first hue value (H1) corresponding to red anda second hue value (H2) corresponding to green, the proportionalityfactor f_(C(H)) is equal to 1and the number of color points in the huedirection in this range is equal to N_(H,RG); wherein, for hue valuesbetween the second hue value (H2) corresponding to green and a third huevalue (H3) corresponding to blue, the proportionality factor f_(C(H)) isset to a constant value f_(GB) such that the number N_(H,GB(fGB)) ofcolor points in the hue direction in this range differs less than 50%from N_(H,RG), N_(H,GB(fGB)) being equal to N_(H,RG); wherein, for huevalues between the third hue value (H3) and the first hue value (H1),the proportionality factor f_(C(H)) is set to a constant value f_(GB)such that the number N_(H,BR(fBR)) of color points in the hue directionin this range differs less than 50% from N_(H,RG), N_(H,BR(fBR)) beingequal to _(N) _(H,RG); and wherein ΔH is a function of H such that, forsteps in the hue direction along the border of the color space, saidperceptual color difference ΔE_(INV) is substantially constant; for eachof the color point thusly defined, calculating corresponding colorcontrol values (ξ1, ξ2, ξ3) and writing those values into the memory. 8.A method for navigating a lamp assembly, capable of generating lightwith a variable color, through a three-dimensional CIELAB color space,the method comprising the steps of: providing a memory according toclaim 7; receiving user input data (m,n,p); responsive to the user inputdata (m,n,p) and on the basis of the information in the memory,generating color control signals (ξ1, ξ2, ξ3) for the lamp assembly,such that the color point of the generated light corresponds to the userdata; and responsive to the user input data (m,n,p), calculating a stepAH in the hue direction, a step ΔL in the lightness direction, and astep ΔC in the chroma direction, or a combination of such steps, takinginto account a current color point, calculating a target color pointbased on said steps ΔH, ΔL, ΔC, and generating the color control signals(ξ1, ξ2, ξ3) for the lamp assembly in conformity with the target colorpoint.
 9. A method for navigating a lamp assembly according to claim 8,further comprising the step of taking from memory data relating to acolor point closest to the target color point, and generating the colorcontrol signals (ξ1, ξ2, ξ3) for the lamp assembly in conformity withthese data from memory.